Conventional technologies of optical fiber communication networks are still confronted with several technical challenges and difficulties to achieve high speed full range wavelength tuning while maintaining wavelength and phase stability as the optical transmissions encounter wavelength dispersions over long distance transmission and operated over greater ranges of temperature fluctuations. There is an ever-urgent demand to resolve these limitations and difficulties. Specifically, in fiber telecommunications, tunable lasers are essential to provide system reconfiguration and reprogramming. Future applications may also require a laser with a higher power to compensate the components losses and a narrower line width to battle with chromatic dispersion. A fiber laser can potentially meet all these requirements. By integrating a tunable filter inside the cavity, the lasing wavelength can be tuned over the range of the tunable filter. However, conventional techniques for such wavelength tunings are still limited by a lower achievable tuning speed not compatible with the requirements of the next generation fiber telecommunication applications.
To achieve a full range wavelength tuning in C and/or L band, the laser suppliers in optical fiber telecommunication are confronted with another technical difficulty of maintaining laser stability while tuning the wavelength. In a fiber laser, the wavelength is tuned with an optical tunable filter (OTF). As the group delay varies with the wavelength in the fiber laser, tuning of wavelength will cause a change of the equivalent cavity length and that in turn causes the instability of the fiber laser. Therefore, the fiber length needs to be controlled by using either a PZT drum with fiber winded on it or a delay line. The speed of tuning and locking a fiber laser is controlled by both the speed of an electronically tunable filter and the speed of fiber length modulation apparatus. However, as the group-delay difference between two tuning wavelengths is increased, the corresponding fiber length adjustment has to increase also and that leads to a reduced tuning speed. For example, an SMF 28 has a maximum group delay difference of 570 ps/km between 1530 nm and 1565 nm (corresponding to relative effective index change of 4×10−6/nm). The maximum relative displacement for a PZT fiber length modulator can only reach to 5×10−5. So, the maximum wavelength tuning range is limited to be 5×10−5/4×10−6 nm=12 nm. Even though a delay line can be used instead, but the tuning speed is usually limited with the driving motor and not practical for a mode locked fiber laser. For the purpose of achieving a full range of wavelength tuning with a PZT drum, a person of ordinary skill in the art is faced with a challenge to keep the maximum normalized group delay difference below 200 ps/km in order to achieve high speed wavelength tuning while maintaining laser stability.
Furthermore, a fiber telecommunication system is operated under conditions with broad ranges of temperature variations. As the temperature changes, the light path variations induced by temperature variations within the fiber cavity will again cause the instability of the laser operation and degrade the laser performance.
Therefore, a need still exists in the art of optical fiber system and component manufacturing and design to provide new and improved system and component configurations and designs to overcome the above-mentioned technical difficulties and limitations.